Felt WAY shorter to me. I beat it in like 5 hours, and that was with exploring.

Are you suggesting that Portal 1 was 10 hours long? Because that's fairly absurd. The first 15 levels of Portal 1 were tutorials that required little to no thought. As such, Portal 1 took about 3 hours to complete. The puzzle difficulty in Portal 2 was more balanced (though still too easy) and the game had a lot more content, resulting in an average playthrough length of 6-7 hours, with 3-4 hours of co-op content.